This invention relates in general to imaging and more specifically to migration imaging and a process for setting, i.e., stabilizing migration imaging electrical latent images.
Recently, a migration image system capable of producing high quality images of high density, continuous tone, and high resolution has been developed. Such migration imaging systems are disclosed in copending applications Ser. No. 837,780, now U.S. Pat. No. 3,975,195, and Ser. No. 837,591, now U.S. Pat. No. 4,013,462, both filed June 30, 1969 which are hereby expressly incorporated herein by reference. In a typical embodiment of the new migration imaging system an imaging member comprising a substrate with a migration layer comprising a layer of softenable material and electrically photosensitive migration material is imaged in the following manner: an electrical latent image is formed on the member, for example, by electrically charging the member and exposing it to a pattern of activation electromagnetic radiation such as light. Where the photosensitive migration material is layered in but spaced apart from one surface of the softenable material layer (the layer configuration), migration material from the migration layer migrates imagewise toward the substrate when the member is developed by softening the softenable layer.
One mode of development entails exposing the member to a solvent which dissolves only the softenable layer. The photosensitive migration material (typically particles) which has been exposed to radiation migrates through the softenable layer as it is softened and dissolved, leaving an image of migrated particles corresponding to the radiation pattern of an original on the substrate with the material of the softenable layer substantially washed away. The particle image may then be fixed to the substrate. For many preferred photosensitive migration particles, the image produced by the above process is a negative of a positive original, i.e., particles deposite in image configuration corresponding to the radiation exposed areas. Those portions of the photosensitive material which do not migrate to the substrate are washed away by the solvent with the softenable material layer. However, positive to positive systems are also possible by varying imaging parameters. As disclosed in the referenced applications, by other developing techniques, the softenable material layer may at least partially remain behind on the supporting substrated with or without a relatively unmigrated pattern of migration material complementary to said migrated material.
In another imaging member embodiment, the migration layer comprises migration material dispersed throughout the softenable material layer in a binder layer configuration.
"Softenable" as used herein is intended to mean any substantially insulating material which can be rendered more permeable to migration material migrating through its bulk. Conventionally, changing permeability is accomplished by dissolving, partially dissolving, melting, and softening as by contact with heat, vapors, partial solvents and combinations thereof.
The term "electrical latent image" and the several variant forms thereof used herein includes the images formed by the charge-expose mode hereof which cannot readily be detected by standard electrometric techniques as an electrostatic image for example of the type found in xerography, so that no readily detectable or at best a very small change in the electrostatic or coulombic force is found after exposure (when using preferred exposure levels); and electrostatic latent images of a type similar to those found in xerography which are typically readily measurable by standard electrometers, that is the electrostatic latent images show a surface potential reading typically of at least about 5 to 10 volts.
"Fracturable" layer or material as used herein, means any layer or material which is capable of breaking up during development, thereby permitting portions of said layer to migrate toward the substrate in image configuration. The fracturable layer may be particulate or semi-continuous in various embodiments of the migration imaging members.
"Contiguous," for the purpose of this invention, is defined as in Webster's New Collegiate Dictionary, Second Edition, 1960; "In actual contact; touching; also, near, though not in contact; adjoining."
In certain methods of forming the latent image, nonphotosensitive or inert, fracturable layers and particulate material may be used to form images, for example, wherein an electrostatic latent image isformed by a wide variety of methods including charging in image configuration through the use of a mask or stencil; first forming such a charge pattern on a separate photoconductive insulating layer according to conventional xerographic reproduction techniques and then transferring this charge pattern to the imaging member by bringing the two layers into very close proximity and utilizing breakdown techniques as described for example, in Carlson U.S. Pat. No. 2,982,647 and Walkup U.S. Pat. Nos. 2,825,814 and 2,937,943. In addition, charge patterns conforming to selected, shaped electrodes or combinations of electrodes may be formed by the discharge techniques as more fully described in Schwertz U.S. Pat. Nos. 3,023,731 and 2,919,967 or by the techniques described in Walkup U.S. Pat. Nos. 3,001,848 and 3,001,849 as well as by electron beam recording techniques, for example, as described in Glenn U.S. Pat. No. 3,113,179.
The characteristics of the images produced are dependent on such process steps as charging, exposure and development, as well as the particular combination of process steps. High density, continuous tone and high resolution are some of the image characteristics possible. The image is generally characterized as a fixed or unfixed particulate image with or without a portion of the softenable layer and unmigrated portions of the layer left on the imaged member.
As a consequence of working on this new migration imaging system, the present invention permits migration imaging latent electrical images to be set, so that the electrically latently imaged migration imaging member may be stored in its latent imaged condition for extended periods of time, for example, for days, months and even years before being developed to cause migration in depth in the softenable layer. Surprisingly, in many cases, setting also permits development to take place in ambient room light which is ordinarily activating for the migration imaging member.